Magnetic head

ABSTRACT

A magnetic head is configured so that an aspect ratio w 0 /h 0  of a cross section of a coil ( 11 ) is set to be 1 to 4, or preferably, approximately 1.5. This configuration suppresses the heat generation in the magnetic head that performs the magnetic field modulation, without impairing the efficiency, thereby making the magnetic head suitable for high-frequency modulation.

TECHNICAL FIELD

The present invention relates to a magnetic head for recording orreproducing information with respect to a recording medium by applying amagnetic field to the same.

BACKGROUND ART

Typical conventional magnetic heads include those for use withmini-disks (hereinafter referred to as MDs). One example of suchmagnetic heads for MDs is disclosed by JP 7(1995)-129908A.

The MD is a kind of perpendicular thermomagnetic recording mediaemploying magneto-optical recording techniques. In the magneto-opticalrecoding techniques, for recording, a medium locally heated by a laserbeam to have a reduced coercive force is magnetized by applying theretoa perpendicular magnetic field modulated according to recording signals,so that perpendicular magnetic domains are formed therein. Thismodulated perpendicular magnetic field is generated by a magnetic head.For reproduction, the rotations of planes of polarization due to theKerr effect of reflected light are detected so that magnetizationdirections of perpendicular magnetic domains are read.

MDs also are required to have higher transfer rates mainly in the casewhere they are used for data, pictures, etc., and this recently resultsin that modulated magnetic fields of higher frequencies are demanded.

The following will describe a configuration and operations of aconventional magnetic head for use in the magneto-optical recordingtechnique, while referring to FIG. 8, which is a cross-sectional view ofprincipal parts of the magnetic head. In FIG. 8, 1 denotes a recordingmedium such as a MD composed of a substrate 1 a, a recording film 1 b,and a protective film 1 c. More specifically, other constituent elementssuch as a sliding film, a reflection film, etc. are included, but theyare omitted herein. The recording medium 1 is moved by a mechanism(spindle motor, etc.), not shown in the drawing, in a directionindicated by an arrow A.

2 denotes an objective lens. The objective lens 2 allows a laser beam Lfrom a light source to pass through the substrate 1 a, and converges andfocuses the same onto the recording film 1 b.

51 denotes a coil as a source of magnetomotive force. 52 denotes amagnetic core made of a soft magnetic material. As a soft magneticmaterial, a ternary-compound-oxide magnetic material such as MnZnferrite or NiZn ferrite is used preferably, which has relativelyexcellent high-frequency characteristics.

The magnetic core 52 is formed in an approximate “E” shape in which onecenter yoke 52 a and two side yokes 52 b are connected with one anothervia a base yoke 52 c, and a coil 51 is wound and fixed around the centeryoke 52 a. A magnetic flux generated by the passage of electric currentthrough the coil 51 is guided by the magnetic core 52, thereby causing aperpendicular magnetic field with an intensity necessary for recordingto be applied to the recording film 1 b. The coil 51 and the magneticcore 52 compose a magnetic head.

With respect to a MD, the magnetic core 52 is kept out of contact withthe protective film 1 c so as to avoid damage due to collision with theprotective film 1 c. However, from the viewpoint of the powerconsumption, the coil 51 preferably is arranged as close to theprotective film 1 c as possible in an acceptable range so that theefficiency of converting the driving current of the coil 51 into themagnetic field applied to the recording film 1 b is increased.

Furthermore, assume that a height and a width on one side of a crosssection of the coil 51 shown in the drawing are h0 and w0, respectively.In order to increase the efficiency of the conversion from the currentof the coil 51 to the magnetic field, in the case where h0×w0 is setconstant, that is, a cross-sectional area occupied by the coil 51 is setconstant, it is more effective that w0/h0 decreases, that is, that thecross section of the coil 51 in the drawing have amore-vertically-elongated shape, in a certain range. This is because thecross section of the coil 51 in a vertically-elongated shape means thata width w1 of a magnetic gap that is a space between the center yoke 52a and the side yoke 52 b decreases, which allows a magnetic resistanceof an entirety of the magnetic head to decrease, thereby improving theefficiency. In the case of the shape shown in FIG. 8, w0/h0 isapproximately 0.5.

Furthermore, in the case where the coil 51 has a cross section in avertically-elongated shape, a mean diameter of the coil 51 decreases,which allows the center of the cross section of the coil 51 to bearranged closer to the center yoke 52 a, thereby improving theefficiency also. In this case, the decrease of the mean diameter of thecoil causes the coil resistance to decrease also, thereby reducing thepower consumption of the coil.

When data are recorded, the coil 51 is modulated by a current accordingto a recording signal so as to generate a magnetic flux. The recordingfilm 1 b is subjected to a modulated magnetic field by the magnetic fluxguided thereto by the magnetic core 52. Here, the laser beam L isconverged and focused by the objective lens 2 on the recording film 1 b,thereby heating the recording film 1 b, causing its coercive force todecrease. Therefore, the record therein before the heating is erased.When the recording medium 1 moves in the arrow A direction, thetemperature of the recording film 1 b drops. As a result, the coerciveforce is recovered, and a modulated magnetic field exerted theretocurrently is recorded.

However, the above-described conventional magnetic head has thefollowing problems. When the modulated magnetic field is applied to themagnetic core 52, losses are produced, and mainly are converted intoheat, thereby causing the temperature of the magnetic core 52 to rise.These losses include an eddy current loss and a hysteresis loss, whichare present independently from a so-called copper loss produced due tothe resistance of the coil per se. Considering the hysteresis loss inparticular, in the case where the material characteristics are assumedto be constant, the loss is considered to be proportional to an integralof a magnetic flux density with a volume.

Since an increase in a modulation frequency leads to an increase in thenumber of hysteresis loops within a unit time, the energy consumptionwithin a unit time increases. This causes the temperature of themagnetic core to rise. Further, since an increase in the dimension of h0leads to an increase in the volume in an area with a high magnetic-fluxdensity, an energy-consuming area of the core increases.

Generally, the magnetic permeability of a magnetic material has atemperature-dependent characteristic; the magnetic permeability abruptlydrops when the temperature exceeds a certain level, and approximates theabsolute permeability of vacuum when the temperature is in the vicinityof a Curie temperature. In other words, as the temperature of themagnetic core rises, a magnetic resistance increases, and a magneticfield is not generated sufficiently. Furthermore, in the case of alow-cost coil employing an insulation coating with a low heatresistance, there is a possibility that the coil could be burnt out byheat generated by the magnetic core. Furthermore, an increase in themagnetic resistance causes the number of interlinkage fluxes todecrease, thereby reducing an inductance of the magnetic head.Accordingly, in the case of a low-cost constant-voltage circuit or thelike, the current passing through the coil increases due to a decreasein an impedance of the magnetic head, thereby generating further moreheat, and sometimes causing a so-called thermal runaway state, whichburns out the coil, and breaks down the circuit, etc. Consequently, theabove-described conventional magnetic head, which is designed only forreducing an absolute amount of current, has a drawback in that themagnetic head is not suitably employed for high-frequency magnetic-fieldmodulation recording.

DISCLOSURE OF THE INVENTION

The present invention has been made to solve the aforementionedproblems, and it is an object of the present invention to provide amagnetic head that is capable of performing high-frequencymagnetic-field modulation by employing easy and low-cost means.

To achieve the foregoing object, the present invention is configured asfollows.

A magnetic head of the present invention includes a magnetic core and acoil for mainly applying a modulated magnetic field to a medium. In thecase where, in an approximate rectangular-shape cross section of thecoil, a dimension of the same in an excitation direction of the coil isexpressed as h0, and a dimension of the same in a directionperpendicular to the excitation direction is expressed as w0, a ratio α0(≡w0/h0) is set so as to satisfy 1≦α0≦4.

By setting the ratio so as to satisfy 1≦α0≦4, losses at the magneticcore can be reduced without impairing the magnetic field generationefficiency with respect to the current. Therefore, when thehigh-frequency modulation is performed, excellent characteristics can beobtained even if a low-cost magnetic core and coil are used, and thesecan be used in a temperature range with high reliability. Consequently,it is possible to provide an excellent magnetic head that provides bothadvantages of high performance and low cost.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view illustrating a magnetic head accordingto a first embodiment of the present invention.

FIG. 2A is a graph showing how a driving current varies as a ratio α0 isvaried, concerning the magnetic head according to the first embodimentof the present invention, and

FIG. 2B is a graph showing how losses vary as the ratio α0 is varied,concerning the magnetic head.

FIG. 3 is a graph showing how the power consumption and the temperaturerise at a magnetic core vary as the ratio α0 is varied, concerning themagnetic head according to the first embodiment of the presentinvention.

FIG. 4 is a perspective view illustrating a magnetic core employed in amagnetic head according to a second embodiment of the present invention.

FIG. 5 is a perspective view illustrating a magnetic core employed in amagnetic head according to a third embodiment of the present invention.

FIG. 6 is a cross-sectional view illustrating a magnetic head accordingto a fourth embodiment of the present invention.

FIG. 7 is a perspective view illustrating a magnetic core employed inthe magnetic head according to the fourth embodiment of the presentinvention.

FIG. 8 is a cross-sectional view illustrating a conventional magnetichead.

DESCRIPTION OF THE INVENTION

The following will describe embodiments of the present invention, whilereferring to FIGS. 1 to 7.

First Embodiment

FIG. 1 is a cross-sectional view illustrating principal parts of amagnetic head according to the first embodiment of the presentinvention. A recording medium 1 and an objective lens 2, includingdetails thereof, are identical to those of the conventional exampleshown in FIG. 8, and they function in the same manner. The sameconstituent members as those shown in FIG. 8 are designated with thesame reference numerals, and detailed descriptions of the same areomitted herein.

A coil 11 and a magnetic core 12 are identical to those of theconventional example shown in FIG. 8 except for their shapes. The coil11 is wound and fixed around a center yoke 12 a of the magnetic core 12.Side yokes 12 b are provided on both sides of the center yoke 12 a, inparallel with the center yoke 12 a. The center yoke 12 a and the sideyokes 12 b are integrated via a base yoke 12 c, so as to compose themagnetic core 12.

The magnetic core 12 is in an “E” letter shape, and has an approximatelyconstant thickness (the thickness in a direction perpendicular to theplane of the paper carrying FIG. 1). The coil 11 may be fixed to themagnetic core 12 by, for instance, bonding, etc., or alternatively, thecoil 11 may be wound around a bobbin (not shown) that is attached byinsertion fixedly to the center yoke 12 a. The coil 11 and the magneticcore 12 compose a magnetic head.

In FIG. 1 showing a cross section taken along a plane containing acentral axis (not shown) of the winding of the coil 11, which isapproximately parallel to a magnetic flux passing through the centeryoke 12 a, h0 and w0 denote a height and a width on one side,respectively, of the cross section of the coil 11, as in theconventional example. More specifically, h0 and w0 denote a dimension ofthe coil 11 in a direction parallel to an excitation direction of thecoil 11 and a dimension thereof in a direction perpendicular to theexcitation direction, respectively.

Furthermore, h1 denotes a length of the center yoke 12 a, and w1 denotesa distance between the center yoke 12 a and the side yoke 12 b on oneside.

Here, the following will describe a case where w0 ×h0 substantiallyremains constant, that is, a cross-sectional area occupied by the coil11 remains constant. This substantially is equivalent to a state inwhich the number of turns is constant in the case where a wire diameterof the coil 11 is set to be a certain value, and this means a conditionin which a self-inductance is limited to a substantially constant value,which is important for implementing the high-frequency modulation.

For convenience in writing, a dimension ratio w0/h0 of thecross-sectional shape of the coil is defined to satisfy w0/h0≡α0.

FIGS. 2A and 2B illustrate how the characteristics of the magnetic headat a certain specific modulation frequency vary as α0 is varied whilew0/h0 of the coil 11 is set constant. As α0 increases, that is, in theright-side regions of the graphs, the coil becomes a so-called thinnercoil. The shape of the magnetic core 12 also is varied according to thevariation of the shape of the coil 11. In other words, when a dimensionratio w1/h1 is defined to satisfy w1/h1≡α1, α1=α0 is satisfied.

FIG. 2A illustrates how a driving current amplitude of the coil 11needed for obtaining a necessary intensity of the modulated magneticfield varies according to α0. The graph shows that, at least in a rangesatisfying α0>0.5, more current is needed for obtaining a necessarymagnetic field as α0 increases. From the viewpoint of the currentefficiency, in the region shown in the drawing, a smaller α0 is moreadvantageous. This is as shown in conjunction with the conventionalexample.

FIG. 2B illustrates how the resistance (real part of an impedance) ofthe magnetic head in a modulation band varies with α0. Here, theresistance is separated into a coil loss portion and a core lossportion. The total resistance exhibits variation in a hyperbolic curvewith respect to α0, increasing as α0 decreases and substantiallyremaining constant when α0>3. In the breakdown of the losses, to stateroughly, the core loss is greater when α0<1.6, and the coil loss issignificant when α0>1.6.

In other words, as predicted, as h0 increases, that is, α0 decreases,the core loss significantly increases since a volume in an area with ahigh magnetic-flux density increases. In contrast, as α0 increases, thecoil loss increases since a total length of the coil increases. Itshould be noted that the core loss has a great sensitivity to thevariation of α0, while the variation of the coil loss is relatively dullto the same.

FIG. 3 is a graph illustrating the relationship of the power consumed atthe magnetic head and the temperature rise at the magnetic core withrespect to α0. Here, the power is a product of a square of the drivingcurrent shown in FIG. 2A and the total resistance shown in FIG. 2B.

The power and the temperature rise at the core exhibit substantiallysimilar curves with respect to α0, and the both have minimum values whenα0 is in the vicinity of 1.9. This is because when α0 is small, thepower increases since the loss at the magnetic core increases, and whenα0 is large, the power also increases since the driving current and theloss at the coil increase. Therefore, from the viewpoint of the power,the points of the minimum values appear midway therebetween.

As to portable apparatuses, it is not unusual that they are used at anambient temperature of approximately 40° C., and the temperature insidethe apparatuses occasionally rises to at least 20° C. higher than that,namely, approximately 60° C. In this case, for instance, in the casewhere the coil 11 is coated with polyamide from the viewpoint of thecost reduction, a critical temperature for heat resistance thereof isapproximately 100° C., and therefore, an acceptable temperature rise is+40° C. A selectable range of α0 in this case is 1≦α0≦4 as seen in FIG.3.

Furthermore, likewise, assuming that a temperature rise margin isapproximately 10° C., the acceptable temperature rise is +30° C., andtherefore, the selectable range of α0 is 1.5≦α0≦2.5.

It should be noted that a smaller driving current is preferredconsidering the power for the entire system. As shown in FIG. 2A, thedriving current monotonically increases with respect to α0, and hencethe minimum α0 in the selectable range should be selected. Therefore, inthe case where the acceptable temperature rise is +30° C., an optimalsolution derived by taking the entirety into account could be such thatα0 is approximately 1.5. FIG. 1 illustrating the present embodiment isdrawn assuming that α0≈1.5, based on the foregoing technical idea.

Since α0 in FIG. 8, described in conjunction with the conventionalexample, is approximately 0.5, the temperature rise is approximately 55°C. according to FIG. 3. Accordingly, when a temperature inside anapparatus is 60° C., the magnetic core has a temperature of 115° C.Generally, a magnetic core material having a greater magneticpermeability has a lower Curie temperature, and there are magnetic corematerials with Curie temperatures at a level of 110° C. Therefore, itcan be determined easily from FIG. 3 that the configuration of theconventional example could cause the deterioration of performance of themagnetic head and the thermal runaway.

In contrast, by applying the present embodiment, the temperature rise atthe magnetic core can be avoided readily. Therefore, even in the casewhere a low-cost magnetic core material with a high magneticpermeability and a low-cost coil are employed, they can be used in anappropriate temperature range, and the deterioration of performance ofthe magnetic core as well as the thermal runaway can be avoided.

The operations of the magnetic head according to the present embodimentare identical to those of the conventional example, and hence theirdescriptions are omitted herein.

It should be noted that in the present embodiment, though a wirematerial for the coil 11 is not specified in particular, a Litz wireformed by stranding a plurality of coated thin wires may be used, so asto obtain an improved Q-value, as well as a resonance point at a higherfrequency. This is because the skin effect can be used effectively, andthe capacitive coupling between adjacent wires can be reduced.Consequently, it is possible to decrease the coil loss at a highfrequency, thereby providing a magnetic head with improved performance.

Second Embodiment

FIG. 4 is a perspective view illustrating only a magnetic core of amagnetic head according to the second embodiment of the presentinvention. A magnetic core 22 differs from the magnetic core 12 used inthe first embodiment only in the shape, and is identical to the same inthe materials, etc. On a base yoke 22 c in a rectangular plate shape, acenter yoke 22 a with a height h1 is provided substantially at thecenter, and two side yokes 22 b are provided at symmetrical positions ata distance w1 each from the center yoke 22 a, so as to protrude from thebase yoke 22 c. Though not shown, a coil is wound and fixed around thecenter yoke 22 a in the same manner as that in the first embodiment.Thus, a magnetic head is formed.

The present embodiment is supported by substantially the sametheoretical background as that of the first embodiment, and a ratioα1(=w1/h1) as to the magnetic core 22 and a ratio α0(=w0/h0) as to thecoil, not shown, are set to be approximately 1.5 each.

A width tB of the base yoke 22 c in a direction orthogonal to both thedirection of the height h1 and the direction of the distance w1 isgreater than a width tC of the center yoke 22 a in the same direction.

In the present embodiment, since the base yoke 22 c is in the plate formand satisfies tB>tC, the magnetic resistance further decreases ascompared with the first embodiment, and hence, the efficiency increasesby several percents.

Third Embodiment

FIG. 5 is a perspective view illustrating only a magnetic core of amagnetic head according to the third embodiment of the presentinvention. A magnetic core 32 differs from the magnetic core 32 used inthe second embodiment only in the shape, and is identical to the same inthe materials, etc. On a base yoke 32 c in a rectangular plate shape, acenter yoke 32 a with a height h1 is provided substantially at thecenter. Two side yokes 32 b are provided at symmetrical positions at adistance w1 each from the center yoke 32 a in an x direction shown inthe drawing (direction parallel to one side of the base yoke 32 c), twoside yokes 32 b are provided at symmetrical positions at a distance w1each from the center yoke 32 a in a y direction (direction orthogonal tothe x direction), and further, at each corner of the base yoke 32 c, aside yoke 32 b is provided. Though not shown, a coil is wound and fixedaround the center yoke 32 a in the same manner as that in the firstembodiment. Thus, a magnetic head is formed.

In the present embodiment, a ratio α1(=w1/h1) as to the magnetic core 22and a ratio α0(=w0/h0) as to the coil (not shown) are set to beapproximately 1.5 each, as substantially identical to the secondembodiment.

In the present embodiment, since eight side yokes 32 b in total arearranged so as to surround the center yoke 32 a, the magnetic resistancefurther decreases, thereby further improving the efficiency, as comparedwith the second embodiment.

Fourth Embodiment

FIG. 6 is a cross-sectional view illustrating principal parts of amagnetic head according to the fourth embodiment of the presentinvention. A coil 41 is the same as the coil 11 in the first embodiment.A magnetic core 42 differs from the magnetic core 12 in the firstembodiment only in the shape, and is identical to the same in thematerials, etc.

FIG. 7 is a perspective view illustrating an overall shape of themagnetic core 42. A center yoke 42 a is provided substantially at thecenter of a base yoke 42 c in a rectangular plate shape, so as toprotrude therefrom. A coil 41 is wound and fixed around the center yoke42 a in the same manner as that in the first embodiment.

The present embodiment also is supported by substantially the sametheoretical background as that of the first embodiment, and a ratioα0(=w0/h0) as to the coil 41 is set to be approximately 1.5. Therefore,an excellent magnetic head suitable for high-frequency modulation isprovided.

Thus, the foregoing embodiments are described by taking as an examplethe case where a MD is used as a recording medium, but the presentinvention is not limited to this. The present invention is applicable toany apparatus or medium for use with a magnetic head that performs thehigh-frequency magnetic field modulation.

Furthermore, it is possible to configure the magnetic head so that oneside yoke is provided, according to an apparatus used.

Furthermore, in conjunction with the foregoing embodiments, the caseswhere the side yokes have the same height as that of the center yoke areshown in the drawings, but it also is possible to make the side yokesslightly lower in height than the center yoke, according to a design.

Furthermore, each base yoke in the second to fourth embodiments isrectangular in the plan-view shape, but certain notches or grooves forthe leading out or fixing of the coil wires or other purposes may beprovided, and this does not constitute a deviation from the scope of thepresent invention.

The embodiments described above merely intend to clarify technicaldetails of the present invention and the present invention should not beinterpreted as being limited to such specific examples. The presentinvention can be carried out by being modified variously within a rangeof claims and without departing from its spirit and should beinterpreted broadly.

1. A magnetic head comprising a magnetic core for mainly applying amodulated magnetic field to a magneto-optical recording medium, and acoil wound around the magnetic core, wherein the modulated magneticfield generated by modulating a current flowing through a coil inaccordance with a recording signal is applied locally to a portion ofthe magneto-optical recording medium in which a coercive force isreduced by irradiation of a laser beam, so that informationis recordedon the magneto-optical recording medium, and wherein a ratio α0satisfies:1≦α0≦4 where α0≡w0/h0, and h0 and w0 represent dimensions of across-sectional area of the coil taken along a plane substantiallyorthogonally crossing a direction in which charges move when current ispassed through the coil, h0 and w0 representing a dimension of thecross-sectional area in an excitation direction of the coil, and adimension of the same in a direction perpendicular to the excitationdirection, respectively.
 2. The magnetic head according to claim 1,wherein the ratio α0 satisfies:1.5≦α0≦2.5.
 3. The magnetic head according to claim 1, wherein themagnetic core includes a center yoke arranged so as to face the medium,a side yoke provided substantially in parallel with the center yoke, anda base yoke connecting the center yoke and the side yoke at their endson a side opposite to a side facing the medium, the coil is wound aroundthe center yoke, and a ratio α1 satisfies:1≦α1≦4 where α1≡w1/h1, and w1 represents a distance between the centeryoke and the side yoke, and h1 represents a distance from a surface ofthe center yoke facing the medium to the base yoke.
 4. The magnetic headaccording to claim 3, wherein the ratio α1 satisfies:1.5≦α1≦2.5.
 5. The magnetic head according to claim 3, wherein the baseyoke is in an approximate plate shape having a normal directed in an h1direction in which the distance h1 extends, and a maximum width of thebase yoke in a t direction is greater than a width of the center yoke inthe same direction, the t direction being a direction orthogonallycrossing both of the h1 direction and a w1 direction in which thedistance w1 extends.
 6. The magnetic head according to claim 1, whereina wire of the coil is a multifilament stranded wire.
 7. The magnetichead according to claim 1, wherein a vicinity of a coil-wound portion ofthe magnetic core is in an approximate “E” shape.
 8. The magnetic headaccording to claim 1, the magnetic core includes one center yoke and aplurality of side yokes, and the magnetic core has a cross section in anapproximate “E” shape taken along a plane that contains the center yokeand the side yokes and that substantially is parallel to a magnetic fluxpassing through the center yoke.